The continuous casting of slabs, and particularly stainless steel slabs, is typically accomplished by using a flow-through continuous casting mold having a rectangular internal mold cavity. A submerged nozzle is used for introducing molten metal below the surface of a molten metal pool which is formed in the continuous casting mold. For this purpose, bifurcated submerged nozzles are used; however, these cause problems in the casting operation, particularly in the casting of stainless steel slabs.
Specifically, in the production of stainless steels it is common to add titanium for stabilization purposes. The titanium is added in the tap ladle prior to the continuous casting operation. A portion of the titanium reacts with the nitrogen dissolved in the metal to form small, insoluble nitride particles in the molten metal introduced to the continuous casting mold. These nitride particles tend to coalesce and collect in the continuous casting mold by floating on the surface of the molten metal in the mold or accumulating as entrapped particles in the solidified metal portion of the continuous casting. These nitrides result in objectionable titanium streaks on the surface of the hot-rolled band produced from the continuously cast slab. This may be sufficiently severe to cause rejection and ultimate scrapping of the metal.
Another problem encountered with conventional submerged nozzles occurs during the initial filling of the continuous casting mold with molten metal during startup. During this operation, a considerable quantity of the metal introduced to the mold is initially splashed onto the mold walls. This splashed metal solidifies on the mold walls and becomes oxidized before the molten metal level rises to cover and melt them. This may result in poor surface quality of the initial portion of the slab casting, which ultimately results in surface defects, such as laps and seams, on the hot-rolled band produced from this initial portion of the casting. To prevent this, the mold is initially lined with a metal liner, termed "splash cans" which is designed to prevent metal splashing onto the mold wall surfaces until the metal level in the mold covers the nozzle ports. Thereafter, the splash can melts into the molten metal pool within the mold. Often, however, the splash can melts or otherwise disintegrates before the nozzle ports are covered and thus does not satisfactorily perform its intended function.
Attempts have been made by others, such as shown in U.S. Pat. Nos. 3,517,726, issued June 30, 1970, and 3,578,064, issued May 11, 1971, to use multiport submerged nozzles for continuous casting of slabs. Those patents do not teach or suggest the nozzle of the present invention.